This invention relates to a method of protecting metal with a nontoxic, relatively insoluble, inorganic, corrosion resistant coating. More particularly, the invention relates to a metal sheet having a silane treated aluminate coating. The coating is formed by a two step process including sequentially rinsing the sheet with an alkaline solution containing a dissolved aluminate and another solution containing a hydrolyzed organofunctional silane.
U.S. Pat. No. 5,108,793; incorporated herein by reference, relates to a two step process for forming a steel sheet having a silane treated inorganic silicate coating. The coating is formed by rinsing the sheet in an alkaline solution having a temperature of at least 25.degree. C. containing 0.005M metal salt. The sheet is dried to form a silicate coating having a thickness of at least 2 .ANG. prior to being treated with an aqueous solution containing 0.5-5 vol.-% silane. The silane treated silicate coating provides good corrosion protection on cold-rolled and metallic coated steel when the steel is pretreated with a phosphate conversion coating. For painted steels, the silane film forms an adherent bond between the paint and the silicate coating. When a phosphate conversion coating is not applied, however, the silicate coating may be brittle and have inferior adhesion to the steel.
It also is known to protect galvanized or cold-rolled steel with a silane treated alumina, silica and/or zirconia hydrate. The hydrate coating is formed by immersing the steel into a bath containing a suspension of the oxide particles. Such a hydrate coating generally is a thick, brittle coating that does not provide good paint chipping resistance or good formability. Another disadvantage is non uniformity of the coating because the rinsing solution must be constantly stirred to maintain the oxide particles in suspension.
As evidenced by the effort of previous workers, there has been a long felt need to develop a corrosion resistant coating for cold-rolled steel sheet as well as metallic coated steel sheet that has good paint chipping resistance and good formability. There also has been a need to develop a low cost corrosion resistant coating formed using environmentally safe coating solutions that can be disposed of inexpensively.